Does Manning’s equation work for pipes, not just “open” channels?Yes! As I’ve previously discussed, Manning’s equation can also be used for pipes, as long as there is a free, exposed water surface. The area and wetted perimeter are hard to calculate, but doable if you utilize the graphs that relate circular pipe ratios (D/Dfull, Q/Qfull, etc.). Here’s Manning’s equation below:

(English Units)

Note: If the pipe is pressurized, then Manning’s equation should not be used, but there’s one exception. You can actually use Manning’s equation for a pipe flowing just full, but not technically pressurized. The assumption is that the pipe has just barely become full, and that any additional infinitesimal flow would make the pipe pressurized. The discharge of the pipe in this condition is usually called Qfull or Qfull capacity. Qfull is actually extremely important, because a pipe flowing under the Qfull condition has less discharge than a pipe flowing just below the full depth (a circular pipe conveys the most flow at about 94% of its full depth). The reason why Qfull is less than Q94% depth is because even though there is more flow area in the full-condition, there is even more friction (wetted perimeter) gained as a result of the pipe closing in on itself. This additional friction cancels out the additional flow area and slows down the water.

Take a look at this example to see how this concept applies to a nine foot pipe:

If you still can’t believe this is true (because I definitely didn’t at first!), check out this graph and look for where Q/Qfull is maximum:

Note: This graph assumes “n” does not change with depth.

Qfull is actually pretty usefull (pun intended). At work, I use a popular program called Flowmaster to calculate Qfull. During the beginning, planning stages of sizing a pipe, I’ll use Qfull to get an idea of my pipe’s maximum capacity rather than Q94% depth. Qfull is a safer number to use since there’s always a chance the pipe will seal up with water, especially if there’s a clog in the system or backwater effects.

But remember, gravity-drained systems, such as storm drains, should not be designed solely on the basis of Qfull. A more detailed hydraulic analysis, utilizing the energy equation and a whole lot of iterative calculations (standard-step method) is usually needed, especially if there are any transitions to different-sized pipes, tight curves, abrupt changes in the slope, and/or the pipe becomes pressurized.

As a shortcut for the PE exam, here’s the formula for calculating Qfull in a circular stormwater or sewage pipe. If the pipe is not full, use the circular pipe ratio graphs to calculate A and R for use in Manning’s equation:

(Circular pipe; English Units)

(English Units)

Manning’s equation is perhaps the most popular formula for open channel flow. You can calculate the flow and velocity (Q/A) of a channel or non-pressurized conduit, such as a circular pipe, using this equation.

This formula can also be rearranged to solve for the normal depth (yn) of an open-channel, such as a rectangular channel:

(rectangular channel; solve by trial & error)

Here’s a summary of each term below:

Q: Flow, a.k.a. discharge (cfs)

n: The Manning’s “roughness” coefficient of the channel. This value shows how much resistance is acted upon the water by the channel. A lower n-value means less roughness, and usually implies a higher velocity and smaller depth in the channel (and vice-versa), with all else being equal. Concrete, which is valued for its hydraulic “smoothness”, has an n-value between 0.013-0.015. For comparison, a natural stream with little to heavy vegetation can have an n-value ranging anywhere between 0.025 to 0.150. Now just imagine riding your road bike on concrete vs. a grassy field, and which surface is much easier to ride on. That’s how the water feels.

A: Flow area (ft2) (Note: not necessarily the full area of your channel section!). For example, if a rectangular channel is flowing half-full, the area would be the base x ½ height, not base x height.

w: width (base) of a rectangular channel (ft).

R: Hydraulic radius (ft), or R = A / P. P is the wetted perimeter, or the length of water that is in contact with the physical channel (i.e. receiving friction). For example, in a rectangular channel flowing half-full, wetted perimeter is the base plus the length of both vertical sides touching the water (see equation above).

S: Technically it’s the friction slope (Sf), but for most applications (and on the P.E. exam) it is the channel’s slope, in decimal form (e.g 0.003 or 0.2). Channel slope is assumed because the prime assumption of Manning’s equation is that the channel is flowing under uniform flow. In uniform flow, the gravitational forces (i.e the weight of the water) cancel out the frictional (resisting) forces, which causes the friction slope to equal the channel slope (Sf = Schannel). Do an energy balance calculation between two points on a uniform-flow channel and prove it to yourself (I will cover this in a more nerdy, in-depth discussion of uniform flow in the near future)

yn: Normal depth (ft), or the depth of flow the water would normally take in the channel assuming there are no changes in the channel’s shape, friction, or backwater effects in either the upstream or downstream direction from the channel for a good distance. In other words, this is uniform flow, as will be discussed in a future post.